Spotted this long worm with a flat, shovel-shaped head slithering on the patio stones. It looks totally unnatural. What is this?

1. Meet the Hammerhead Worm: The Strange Visitor in Your Yard
The hammerhead worm, belonging to the genus Bipalium, is a land flatworm known for its head, which resembles the shape of a hammerhead shark. These worms can grow surprisingly long—typically between 8 and 12 inches (20 to 30 centimeters), though some species exceed that length. Their bodies are flat and coated in slime, usually brown or gray, often marked with darker stripes running lengthwise.

Unlike earthworms, which improve soil health, hammerhead worms are carnivores. They feed on other invertebrates, especially earthworms. They also reproduce asexually through fragmentation: if a worm breaks into pieces, each fragment can regenerate into a new individual. This ability allows them to spread quickly and establish populations in new areas.